Addition of CNR (R = Me or But) to the MM complex [{M(CO)2(η-C5H5)}2](M = Mo or W) results in addition of one isonitrile across the MM bond and the formation of [M2(µ-η2-CNR)(CO)4(η-C5H5)2]. The But molybdenum derivative crystallises as black platey needles [orthorhombic, a= 31.75(3), b= 8.171(11), c= 15.722(17)A; R converged to 0.0357 for 1 775 independent reflections for which I/σ(I) > 3.0]. The molecule consists of two Mo(CO)2(η-C5H5) groups linked by a Mo–Mo single bond and bridged by the CNBut group in a µ-η2 four-electron donor fashion. The bonding is viewed as donation of the carbon long pair to one molybdenum atom and donation of CN π-electron density to the second. Prolonged reaction times of [Mo2(µ-η2-CNR)(CO)4(η-C5H5)2] with excess CNR results in cleavage of the CN–Mo π-donor interaction and formation of the symmetrically disubstituted complex [{Mo(CNR)(CO)2(η-C5H5)}2]. The But derivative crystallises as dark red prisms [monoclinic, a= 10.354(7), b= 11.672(5), c= 10.793(5)A, β= 103.68(4)°; R converged to 0.0519 for 1 102 independent reflections for which I/σ(I) > 3.0]. The molecule consists of two Mo(CNBut)(CO)2(η-C5H5) groups linked through a Mo–Mo single bond with a crystallographically imposed centre of symmetry midway along the Mo–Mo bond. Decarbonylation by thermolysis of this compound results in the formation of [Mo2(CNBut)(µ-η2-CNBut)(CO)3(η-C5H5)2], containing one terminal and one bridging isonitrile. Reductive cleavage of [{Mo(CNR)(CO)2(η-C5H5)}2] with sodium amalgam gives the anions [Mo(CNR)(CO)2(η-C5H5)]– which react with SnPh3Cl to give the tin complexes [Mo(SnPh3)(CNR)(CO)2(η-C5H5)], as mixtures of cis and trans isomers.